Urera kaalae is a long-lived perennial shrub or small tree in the Urticaceae (nettle) family. It is a small tree or shrub 3 to 7 m (10 to 23 ft) tall. The sap of the plant becomes greenish black when exposed to air. Leaves are pale green, thin and membranous, heart-shaped, and 10 to 27 cm (4 to 11 in) long by 5 to 13 cm (2 to 5 in) wide, with three main veins and toothed margins. Flowers are either male or female and may grow on the same or different plants. They are arranged in three-branched inflorescences. Sepals of male flowers are fused into rather globe-shaped structures about 1.5 mm (0.06 in) long. Sepals of female flowers are less than 1 mm (0.04 in) long, and the inner pair becomes slightly fleshy to enclose the achene along about half of its 1 mm (0.04 in) length. This species can be distinguished from the other Hawaiian species of the genus by its heart-shaped leaves.